The present invention relates to gaskets for protecting electronics equipment from electromagnetic interference.
There is often a need to protect electronics equipment from electromagnetic interference (EMI) caused by background electromagnetic radiation generated by other electrical equipment, telecommunications broadcasts, electrical discharges and the like, and it is common to provide electronics equipment with EMI gaskets for this purpose. The EMI gasket should clearly be electrically conductive and is therefore usually made from metal, but should also be deformable so that it will take up the profile of whatever component is being placed in contact with it so as to reduce the size of any gaps that may occur between the gasket and the component and therefore reduce the passage of electromagnetic radiation through such gaps.
Many designs of EMI gasket have been made for this purpose, and the gaskets may be continuous or discrete, the discrete gaskets being designed to fit a defined size and shape of aperture in an electronics module or assembly, while the continuous gaskets are xe2x80x9ccut-to-lengthxe2x80x9d designs in which a length of a gasket strip is cut off and attached to the equipment around any aperture to be protected.
This invention is primarily concerned with continuous EMI gaskets. One form of gasket that is available on the market is so-called xe2x80x9cEMI finger stripxe2x80x9d in which a length of metal strip is formed having an array of transversely extending slits along a longitudinally extending central region. Longitudinally extending lateral edge regions may be folded under the central region, and the central region may be bent, along the length of the strip, to give the strip an arcuate cross-sectional profile. This form of gasket is very flexible about its transverse axes because the transversely extending slits extend over substantially the entire width of the gasket (once the edge regions have been folded underneath the central region) so that the slits can open out to accommodate bending of the strip.
This form of EMI gasket can be very effective for reducing levels of unwanted electromagnetic radiation once the various components of the assembly have been located in place, but it can be difficult in certain circumstances to locate the various parts of the assembly with respect to one another about the gasket. No major problem is normally encountered if one part of the assembly is moved into position in a direction perpendicular to the gasket so that it is moved until it comes into contact with the gasket and a small degree of deformation of the gasket will allow the component to be seated correctly with respect to the gasket. However, problems often occur where one of the parts of the assembly must be moved along the gasket during installation. For example, where one module of an assembly must be inserted into a recess in a partition wall, and finger strip is located on the two opposing edges of the recess along which the module slides during insertion, the leading edges of the module will often catch or snag on the transverse slits in the finger strip, thereby reducing the effectiveness of the gasket at best or at worst damaging the gasket.
For this reason a somewhat different version of EMI gasket is offered. In this version of gasket no transverse slits are provided at the central region of the gasket (forming the apex of the gasket in transverse cross-section) and transverse slits are instead provided at the sides of the gasket. This form of gasket is less flexible than the finger strip, but it does allow components to be inserted into the assembly by sliding them along the gasket. As with the finger strip, one or both longitudinally extending lateral edge portions may be folded under the central portion and an attachment arrangement for attaching the gasket to the assembly or component may be enclosed within the gasket so formed. Such a form of gasket will allow, in many cases, insertion of a module into an assembly by sliding it along the gasket. However, a problem still exists by virtue of the abrupt start of the gasket. The profile of the gasket does not change along its length and at each end the gasket starts abruptly at its full height. This can still therefore cause snagging of the leading edge of a component such as a module on the front end of the gasket with the result that the gasket may buckle as the component is inserted. Alternatively, the strip forming the gasket may be pushed off any attachment arrangement that is used to retain it on the electronics arrangement.
According to one aspect the invention provides an elongate gasket that can be placed on an electrical assembly in order to reduce electromagnetic interference, which comprises
(i) an elongate deformable strip of electrically conductive material that has been folded along its length to form an arcuate cross-sectional profile; and
(ii) an attachment arrangement to secure the gasket onto the component;
wherein at least one of the strip and the attachment arrangement is folded at an end region thereof in order to provide a smoothly varying longitudinal profile at the end of the gasket.
The deformable strip or the attachment arrangement may be folded about the other at one end of the gasket only or at both ends depending on circumstances.
The gasket according to the invention has the advantage that at at least one end region, the longitudinal profile of the gasket will vary smoothly from a fraction of its full height to its full height. Accordingly, it becomes considerably easier to insert a component into the assembly at one end of the gasket and slide the component along the gasket without the leading edge butting up against the front end of the gasket.
In one aspect of the invention, it is possible for the front end of the attachment arrangement (which may itself be in the form of a strip) to be folded over the top of the deformable strip in order to generate the smoothly varying profile. Such an arrangement will leave a free end of the attachment arrangement located on top of the deformable strip and pointing in the direction of insertion of the component. Although this design is not optimal, it will allow insertion of a component by sliding it along the gasket without any significant difficulty. Removal of the component by sliding it along the gasket in the opposite direction, i.e. against the direction of the free end of the attachment arrangement, should not present a difficulty either, because the component is being removed and so no edge of the component, other than a trailing edge, passes over the free end of the attachment arrangement.
In another form, however, the end region of the deformable strip is folded around the end of the attachment arrangement. This form has the advantage that the free end of the deformable strip is tucked away under the attachment arrangement and is therefore less likely to catch on any electronics component as it is being inserted or removed. In order to facilitate folding the end region of the deformable strip around the end of the attachment arrangement, it is possible to form a longitudinally extending tongue at at least one end of the strip that is folded around the end of the attachment arrangement. The tongue is preferably formed in the central region of the deformable strip in the transverse direction which forms the apex of the arcuate cross-sectional profile and has been folded underneath the end of the attachment arrangement. With this form of gasket, it is possible for an end region extending for about 1 to 2 cm to be somewhat flattened and so provide a lead-in region for inserting any components along the gasket.
The gasket may, if desired, be formed from a length of conductive material that has at least one longitudinally extending array of transversely extending slits so that when it is folded along its length to give it an arcuate cross-sectional profile, the transversely extending slits occur along one side edge of the gasket and will enable it to be flexed more easily. If the length of conductive material has two parallel arrays of transversely extending slits that are separated from one another by a region of the conductive material that will form the apex of the arcuate transverse profile, then when the length of conductive material has been folded along its length to form the deformable strip, the slits will occur at both sides of the gasket. As an alternative, arrays of oblique slits may be provided.
The strip is usually provided with a base portion that has been formed by folding at least one longitudinally extending lateral edge portion along the length of the strip. Alternatively it may be formed by folding each lateral edge portion toward the other lateral edge portion along the length of the strip so that an attachment arrangement can be held between the two folded edge portions. The attachment arrangement may take any of a number of forms: it may be formed as a continuous length of a strip that can be cut to the appropriate length. Alternatively, it may be formed as number of discrete attachment devices that can be attached to the electronics assembly at predefined points. Whichever form of attachment arrangement is employed it can be attached to the electrical assembly by bonding or mechanically, for example by means of barbs that are pushed through small holes in the assembly. In one form of gasket that is convenient to use, the attachment arrangement is in the form of a continuous strip that can be cut to length and inserted into an end of the deformable strip of material and slid into it. A number of barbs may be located at predetermined positions along the strip so that the gasket can be attached to the assembly after it has been cut to length and the ends folded simply by pressing it against the assembly at points corresponding to the position of the barbs in the attachment strip and holes in the electrical assembly.
Where the base portion has been formed by folding the lateral edge portions, any slits in the sides of the gasket may extend into the lateral edge portions of the strip that form the base portions either partly so that an inner edge region of the base portions remains continuous, or over the entire width of the base portions. In other cases the slits need not extend into the base portion or only to a very limited extent.
The deformable strip needs to be electrically conductive and deformable, and will normally be formed from a metal. As examples, it may be formed from spring stainless steel or beryllium copper. In addition, it is advantageous for it to be resiliently deformable so that it will adopt its original profile when a component is removed from the electrical assembly, and the gasket will often therefore be formed from a material such as berylium copper. Other parts of the gasket, for instance any attachment strip, may be formed from the same material as the deformable strip, or alternatively, and especially where discrete attachment components are employed, may be made from plastics material.
According to another aspect, the invention provides an elongate gasket that can be placed on an electrical component in order to reduce electromagnetic interference, which comprises:
(i) an elongate deformable strip of electrically conductive material that has been folded along its length to form an arcuate cross-sectional profile; and
(ii) an attachment arrangement to secure the gasket onto the component;
wherein the elongate deformable strip can be folded over the end of the attachment arrangement at an end region thereof in order to provide a smoothly varying longitudinal profile at the end of the gasket.
According to yet another aspect, the invention provides an electronics assembly that contains a gasket for reducing electromagnetic interference on an edge or surface thereof, the gasket comprising;
(i) an elongate deformable strip of electrically conductive material that has been folded along its length to form an arcuate cross-sectional profile; and
(ii) an attachment arrangement that secures the gasket to the assembly,
wherein at least one of the strip and the attachment arrangement is folded at an end region thereof over the end of the other of the strip and the attachment arrangement in order to provide a smoothly varying longitudinal profile at the end of the gasket.
According to a still further aspect, the invention provides a method of providing an electronics assembly with a gasket for reducing electromagnetic interference, which comprises attaching a gasket to one part of the assembly, the gasket comprising an elongate deformable strip of electrically conductive material that has been folded along its length to form an arcuate cross-sectional profile, and an attachment arrangement that secures the gasket to the part of the assembly, wherein at least one of the strip and the attachment arrangement is folded at an end region thereof over the end of the other of the strip and the attachment arrangement in order to provide a smoothly varying longitudinal profile at the end of the gasket, locating a second part of the assembly at the end of the gasket and sliding the second part of the assembly along the gasket in the longitudinal direction thereof until the two parts of the assembly are correctly positioned relative to one another.